The preparation of water absorbing polymers is, for example, summarized “Modern Superabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or in Ullmann's Encyclopedia of Industrial Chemistry, 6th ed. vol. 35 pp 73-103. The preferred preparation process is the solution or gel polymerization. When using this technology usually a monomer mixture is prepared which is neutralized and then transferred into the polymerization reactor and is then discontinuously or continuously polymerized to form a polymer gel which is in case of a stirred polymerization comminuted. The polymer gel is subsequently dried, ground and sieved and optionally a surface treatment is applied.
Methods of continuous polymerization are, for example, described in WO-A-01/38402, WO-A-03/004237, WO-A-03/022896 and WO-A-01/16197.
For lab experiments, raw materials of high purity can be used, whereas in production there are limitations with respect to employing highly pure materials. For economical reasons, raw materials such as acrylic acid and caustic of technical grade are preferable employed. Also normal tap water or specifically provided, partially de-mineralized process water, which might be prepared from ground water or other available resources may be preferred over the more costly, fully de-ionized or distilled water.
On production scale it would, furthermore, be desirable to recycle to the monomer solution prior to polymerization scrubber water resulting from the purification of the vent streams of regular production plants and the portion of superabsorbent polymers, which is below specification in terms of particle size (fines). Technical grade raw materials as well as the scrubber water and the fines may contribute to the levels of impurities, which will finally be present in the monomer mixture and will possible disturb the polymerization reaction and may result in an inferior product quality.
Among the various impurities that might be present, heavy metal ions, particularly iron ions, play a significant, disturbing role. Iron is a known very active co-initiator is for redox systems and it has a significant impact on the product quality. U.S. Pat. No. 4,659,793 discloses a process for the preparation of aqueous solutions of copolymers of dicarboxylic acids with α,β-ethylenically unsaturated acidic monomers such as (meth)acrylic acids, whereby in the reaction mixture 1 to 250 ppm of metal ions, particularly iron ions are present. The purpose of the presence of the iron ions is to reduce the content of residual unreacted dicarboxylic acids.
In controlled polymerization reactions for the manufacture of high performance SAP, iron ions are not desired as they may interfere with the initiator system thereby reducing the molecular weight, promoting undesired grafting etc. Iron ions are particularly not desired as their concentration cannot reliably be controlled under standard production conditions. Well designed initiator systems are employed, preferably comprising e.g, the redox couple sodium persulfate and/or hydrogen peroxide and ascorbic acid or other high performance initiator systems in concentrations, which on one hand result in high conversion of the monomer to the polymer and on the other hand allow a high molecular weight, crosslinked product being formed having homogeneous network and low fractions of low molecular weight polymer (extractables). Any additional uncontrolled co-initiator such as iron ions would disturb the precisely tailored system causing undesired effects.
The presence of iron ions causes increased initiation and polymerization rates as it increases the radical concentration. Consequently, the temperature of the reaction mass will rise faster thereby additionally raising the radical formation. Such undesired fast polymerization conditions result in polymers having lower average molecular weights, inferior molecular weight distributions and yet a higher fraction of low molecular weight polymer that will not be linked to the network (extractables). In general, an inferior, more inhomogeneous network is formed during such polymerization conditions, which is not preferable for the manufacture of a high performance superabsorbent polymer.
A further undesired effect of the spontaneous initiation and very high polymerization rate, which is promoted by the presence of iron ions is, that at the location where the co-initiator such as ascorbic acid or its salts, gets in contact with the monomer solution initiation and polymerization is so fast that the ascorbic acid gets trapped into the gel that is thereby formed. The reaction and gel formation is so fast that a homogeneous distribution of this co-initiator over the entire monomer solution is not possible. This inhomogeneous distribution consequently leads to inhomogeneous polymerization.
A further serious problem of such undesired conditions caused by the presence of iron ions in monomer solution is the risk that premature polymerization can occur in the process steps up-stream to the reactor, in particular after de-oxygenation, causing blockages in the preparation conveying system. This can lead to frequent plant shutdowns considerably reducing the commercial efficiency of the process.
WO 03/022896 teaches a continuous polymerization process, wherein the maximum temperature in the reactor is preferably controlled to be below 85° C. and in the initiation zone preferably between 40 to 85° C. In cases of too high initiation and polymerization rates the energy input by the polymerization reaction may be too high so that the temperature cannot be controlled within the preferred range. Too high temperature consequently will support, as has been discussed, a too high radical formation rate leading to even higher initiation and polymerization rates. It is believed that iron ions are the major contributor to such an uncontrollable radical formation rate and that the kinetics of polymerization can no more sufficiently be controlled.
WO 93/05080 suggests to use chelating agent like pentasodium salts of diethylen triamine pentaacetic acids in order to remove trace metals from the reaction mixture. Such chelating agents are used in amounts between about 100 and 2,000 ppm based on the α,β-ethylenically unsaturated monomers. The lowest amount of chelating agent in the examples of WO 93/05080 is 171 ppm based on the monomer mixture. Similarly, WO 03/022896 uses in the examples chelating agents in an amount of about 280 ppm based on the total monomer mixture.
Another group of prior art references EP-A 257 951, U.S. Pat. No. 6,313,231, EP-A-1 108 745, U.S. Pat. No. 6,335,398 and US 2007/141338 deal with the problem of the effect of iron ions in the superabsorbent polymer which may result in combination with ascorbic acid originating from body fluids like urine or blood in a degradation of these superabsorbent polymer in a superabsorbent structure like a diaper. To counteract the effect of iron ions on the degradation of the already formed superabsorbent polymer, these reference suggest to add a chelating agent to a superabsorbent polymer.
But these references only deal with the effect of iron on the already formed superabsorbent polymer in a superabsorbent structure, but do not discuss the effect of iron on the polymerization process and the properties of the superabsorbent polymer.
In general, the prior art teaches the detrimental effect of iron ions either in the preparation of superabsorbent polymers or on the final superabsorbent polymer when coming into contact with body fluids. In any event, the only suggestion obtained from the prior art is that iron ions should be completely removed by considerably high amounts of chelating agents to avoid their detrimental effect.
When investigating the teaching of the prior art to use the chelating agent in the generally disclosed amounts in order to counteract the negative effects of iron ions that after the additional co-initiator an undesired delay of the start of the polymerization of several minutes and an undesired retardation of the polymerization rates was experienced resulting in less than optimal product properties of the resulting superabsorbent polymer. Thus, it is the object of the present invention to avoid the disadvantages of the teaching of the prior art as discovered by the present invention and provide a process for producing a superabsorbent polymer having improved properties.